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' in suitable quantity UNITED STATES PATENT OFFICE.

SAMUEL H. BROWN, OF BOSTON, MASSACHUSETTS, ASSIGNOR OF ONE-HALF TO MICHAEL MOBARRON, OF SAME PLACE.

ANNEALING PROCESS.

SPECIFICATION forming part of Letters Patent No. 468,740, dated February 9, 1892.

Application filed November 11, 1891.

To all whom it may concern:

Be it known that I, SAMUEL H. BROWN, a citizen of the United States, residing in Boston, in the county of Suffolk and State of Masssachusetts, have invented a new and useful Process of Annealing Metals; and I do hereby declare that the following is a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to practice the same.

My invention relates to an improved process of annealing metals-as steel, iron, aluminum, copperan d its alloys, and the precious metals in a new annealing bath and to a process of restoring and maintaining said loath.

Heretofore in annealing metals it has been difficult to bring all of the parts of the article under treatment to a uniform heat and to heat the same to a proper'temperature without oxidizing the surface either during the heating or during the subsequent cooling period.

It is the object of my invention to obviate these difficulties and expeditiously and economically anneal the various metals and their alloys.

To this end my invention consists in subjecting metal to the action of a molten bath of a nitrite of an alkali metal aim'ward SllbJ ectin g the same to the action of a coolingbath.

In practice I subject the metal to be treated to a heat of about 900 Fahrenheit in a bath composed of the nitrite of potassium and afterward withdraw the same and plunge it into a cooling-bath. I take the nitrate of potassium and heat it in a proper vessel to a temperature of from 800to 1,000 Fahrenheit, otherwise to a low red heat, at which temperature it parts Witha portion of its oxygen, which, if allowed to remain, would oxidize the surface of the metal under treatment. The bath is now in a limpid molten condition, ready for the immersion of the articles to be annealed, and is essentially a nitrite of potassium, which only gives off further oxygen slowly, and then in connection with nitrogen. The metal to be treated is now in troduced to the bath, and when it has become heated to about the temperature of the bath' say, in about two minutes, more or less, according to its dimensions and character--it is withdrawn. It will then be found covered I With a coating or film of the material of the bath, which effectually protects it from the Serial No. 411,540. (No specimens.)

oxidizing effects of the atmosphere and keeps it in condition to be subjected to the final cooling step of the annealing process.

In the case of copper, its alloys, or the precious metals the metal or alloy is transferred immediately to the cooling-bath. In the case of steel or iron the material is held with its protective film or coating intact for a short time, when it is immersed in the cooling-bath. The cooling-bath maybe of any suitable material; but I prefer to employ a bath of about forty-eight per cent. of glyceriue, forty-nine per cent. of water, and about three per cent. of starch. When the metal is plunged into the cooling-mixture, the film or coating is re moved and the surface is left smooth and free from scale.

To maintain the volume of the molten bath as it becomes reduced by the withdrawal of portions of the same as a film or coating on the metal inserted and withdrawn additional nitrate of potassium is added in suitable quantities from time to time. This serves to restore and maintain the bath in the condition of a nitrite.

Having now described my invention, what I claim is 1. The process of annealing metals, which consists in fusingnitrite of potassium, raising the temperature of the fused bath to about 900 Fahrenheit, maintaining the bath at this temperature a suitable period to drive off a portion of the contained oxygen, placing the metal in the resulting bath until it becomes sufliciently heated, and finally cooling the metal, as specified.

2. The process of forming and maintaining an annealing-bath of nitrite of potassium, consisting in raising nitrate of potassium to a temperature of about 900 Fahrenheit, maintaining the same at this temperature a suitable period to drive off a portion of the contained oxygen, and adding nitrate of potassium at suitable intervals to the fused nitrite, so as to supply the oxygen necessary to prevent the nitrite from being reduced to an oxide, substantially as described.

In testimony whereof I affix my signature in the presence of two witnesses.

SAMUEL H. BROWN. Witnesses:

WM. M. STOCKBRIDG-E, GOMER J ones. 

